I have looked fairly extensively into the original DSP amplifier to try and find what fails, and why, and see if it is repairable.
I've got about 5 amps here which are all dead, and one which actually still works.
So far what I have found out is that when they fail completely, it appears that the amplifier chips are being held in standby mode and not switching on. I traced the circuitry back to what controlled this and tried replacing a couple of parts, with no success. I even tried bypassing this to force the amp chips into waking up - but even then there was still no audio being processed and coming out.
I've looked into the input circuitry that's used to wake up the DSP chips that do all the work, and that appears to be working as designed. The power supply sections also all seem to be working properly - nothing abnormal when you go to power it up and make it do some work, so no 'under load' faults that I can see.
I have also tried swapping over some of the EEPROM and RAM chips between boards to see if it's something simple like faulty memory that's causing the DSP chip to flag an error when starting up - but again, nothing different.
After everything I have ruled out, it indeed appears to be a fault in at least one of the actual DSP processing chips itself - which are covered in a blob of epoxy on the board. I did try to remove this on one of the faulty boards I had, in case I could get to the chip and find out what it was - but my attempt ended up physically damaging the DSP chip. So unfortunately it appears that once these units have failed, then it's totally dead.
I have also looked into other options, such as swapping in an amp from another vehicle - though the DSP settings on the P38 are somewhat limited - there MAY be a version of a DSP amp from a BMW of around the same era that might work. Some people put the XQK100210 amp from a Disco II in, but this isn't a DSP unit and only some of the speakers work. I looked briefly into whether you could do some rewiring/swapping of pins about to get it to work like a bog standard 8 channel amp (low and mid/high for each door) but the way that the system is set up in the D2 appears to have more low end speakers in it, so then you're into trying to mess about with the components on the board to get the correct audio signals to the proper speakers. That ended up in the too hard basket - which is why I developed the replacement for the DSP amp which has 4x standard P38 door amps on it, and a custom wiring loom which wires into the vehicle wiring, and additional feeds to run to the radio head unit for the inputs for the rear left/right doors. This can also be then used with aftermarket head units with either line balancing transformers on each input, or an attenuator circuit for each input depending on if you are using line level output or amplified output from the replacement head unit.
I haven't had time recently to devote to doing any more with DSP amps or looking into other options, so at the moment I'm still going with the theory that they are unrepairable I'm afraid!
Marty
